You have done all the steps correctly. However the Teiid Designer 7.8 is not compatible with the Teiid 8.1, thus you are seeing the above issues. Teiid Designer tean is soon to release a version that works with Teiid 8.x. Until then you can design the VDBs in the Teiid Designer, but you will not ab able to preview or execute from it. However once you have the VDB, you can deploy into any Teiid server and using tools like SquirreL and issue commands against it.

Also, you can take look at Dynamic VDBs, these are simple XML file way of defining the your virtual database by using DDL. No tooling is involved. Take look at Quick start guide and examples in the downloaded deployment zip.

Also, except for Teiid Designer 7.8 (which works with Teiid 7.7), each version of Teiid has same version denoting the Designer. So, if you are going download prior version use Teiid 7.7 and Designer 7.8.